A RetargetAction tracks the active part in the workbench.
Each RetargetAction has an ID. If the active part provides an action
handler for the ID the enable and check state of the RetargetAction
is determined from the enable and check state of the handler. If the
active part does not provide an action handler then this action is
disabled.

Note: instances of this class add themselves as listeners to their
action handler. It is important for the creator of a retarget action to call
dispose when the action is no longer needed. This will ensure that the
listener is removed.

isHandled

Returns whether this action is handled. In the default case, this is
always true. However, if the action delegates some of its
behaviour to some other object, then this method should answer whether
such an object is currently available.